Transistor half-bridges as well as so-called H-bridges are often used in DC motor driver applications. When switching inductive loads, the driver circuit provides a free-wheeling current path in order to protect the transistors employed in the half-bridges. When using standard MOSFETs as switching transistors a free-wheeling path is provided “automatically” by the intrinsic reverse diode present in the MOSFETs. When using other types of transistors, such as, for example, IGBTs, external free-wheeling diodes may be connected in parallel to the load paths of the transistors. A driver circuit relying only on (intrinsic or external) free-wheeling diodes connected parallel to the transistor load paths implements so-called passive free-wheeling.
When driving inductive loads, for example, with PWM operation, the dissipated power can be significantly reduced by activating the transistor located parallel to the intrinsic free-wheeling diode. A driver circuit that is configured to activate the transistors connected parallel to the free-wheeling implements so-called passive free-wheelings. Active free-wheeling is implemented in present integrated H-bridge driver circuits such as, for example, Infineon's TLE 6209 R and STMicroelectronics' L99H01.
When driving capacitive loads, such as, for example, DC motors having a capacitor connected in parallel, an active free-wheeling may be inappropriate as the capacitor may discharge through the actively enabled transistor current path and consequently passive free-wheeling may be more useful in such situations.
There is a need for versatile driver circuits for controlling the switching states of power transistors arranged in a transistor half-bridge, wherein the driver circuits may make use of the advantages of both types of free-wheeling.